arXiv subjects

Ting-Kuo Lee

Publications and source records attributed to Ting-Kuo Lee.

30 records · Page 2Linked to original sources

arXiv2016

Controversy over large proximity induced s wave like pairing from a d wave superconductor

Proximity effect is used to generate effective topological superconductor by placing strong spin orbit interacting metals with superconducting materials, aiming to produce Majorana zero modes useful for topologically protected quantum computations. In recent experiments high $T_c$ material $\text{Bi}_2\text{Sr}_2\text{CaCu}_2\text{O}_{8+δ}$ is put in contact with a few quintuple layer thick $\text{Bi}_2\text{Se}_3$, and conflicting experimental results are reported. We use standard mean field approach to study this heterostructure, and find it is unlikely to have large proximity induced superconducting gap. The claimed observed s-wave gap might not be purely superconducting coherence gap despite its seemingly correct temperature dependence. Further study on the proximity induced bulk coherence gap and the band structure at the interface should shed light on this issue.

cond-mat.str-el↗

arXiv2015

Genesis of charge orders in high temperature superconductors

One of the most puzzling facts about cuprate high-temperature superconductors in the lightly doped regime is the coexistence of uniform superconductivity and/or antiferromagnetism with many low-energy charge-ordered states in a unidirectional charge density wave or a bidirectional checkerboard structure. Recent experiments have discovered that these charge density waves exhibit different symmetries in their intra-unit-cell form factors for different cuprate families. Using a renormalized mean-field theory for a well-known, strongly correlated model of cuprates, we obtain a number of charge-ordered states with nearly degenerate energies without invoking special features of the Fermi surface. All of these self-consistent solutions have a pair density wave intertwined with a charge density wave and sometimes a spin density wave. Most of these states vanish in the underdoped regime, except for one with a large d-form factor that vanishes at approximately 19% doping of the holes, as reported by experiments. Furthermore, these states could be modified to have a global superconducting order, with a nodal-like density of states at low energy.

cond-mat.str-el↗

arXiv2015

Effective low energy theory for surface Andreev bound states of superconducting phases in generalized Bernevig-Hughes-Zhang models

A method for constructing the low energy effective models for pairings in the generalized Bernevig-Hughes-Zhang model for materials like Bi$_{2}$Se$_{3}$ is proposed. Pairings in this two-orbital model are identified with those familiar in one-orbital models, enabling a unified understanding. The theory provides an easy way to understand the topological nature of the superconducting state that is not directly related to the topological order in the normal state but due to subtle coupling among the degrees of freedom. Furthermore this approach shows a simple way to characterize the anisotropic nature of surface Andreev bound states (SABSs). In particular, we have identified the conditions to have a surprising new result of having two pairs of SABSs. It also leads to a conclusion that SABSs always connect with the topological surface states if the latter are well defined at the chemical potential.

cond-mat.supr-con↗

arXiv2014

Scale-Invariant Dissipationless Chiral Transport in Magnetic Topological Insulators beyond the Two-Dimensional Limit

We investigate the quantum anomalous Hall Effect (QAHE) and related chiral transport in the millimeter-size (Cr0.12Bi0.26Sb0.62)2Te3 films. With high sample quality and robust magnetism at low temperatures, the quantized Hall conductance of e2/h is found to persist even when the film thickness is beyond the two-dimensional (2D) hybridization limit. Meanwhile, the Chern insulator-featured chiral edge conduction is manifested by the non-local transport measurements. In contrast to the 2D hybridized thin film, an additional weakly field-dependent longitudinal resistance is observed in the 10 quintuple-layer film, suggesting the influence of the film thickness on the dissipative edge channel in the QAHE regime. The extension of QAHE into the three-dimensional thickness region addresses the universality of this quantum transport phenomenon and motivates the exploration of new QAHE phases with tunable Chern numbers. In addition, the observation of the scale-invariant dissipationless chiral propagation on a macroscopic scale makes a major stride towards ideal low-power interconnect applications.

cond-mat.mes-hall↗

arXiv2014

Novel anisotropic spin singlet pairings in Cu$_x$Bi$_2$Se$_3$ and Bi$_2$Te$_3$

Possible anisotropic spin singlet pairings in Bi$_2$X$_3$ (X is Se or Te) are studied. Among six pairings compatible with the crystal symmetry, two novel pairings show nontrivial surface Andreev bound states, which form flat bands and could produce zero bias conductance peak in measurements like point contact spectroscopy. By considering purely repulsive short range Coulomb interaction as the pairing mechanism, the dominant superexchange terms are all antiferromagnetic, which would usually favor spin singlet pairing in Bi$_2$X$_3$. Mean field analyses show that the interorbital pairing interaction favors a mixed spatial-parity anisotropic pairing state, and one pairing channel with zero energy surface states has a sizable component. The results provide important new information for future experiments.

cond-mat.supr-con↗

arXiv2013

Intrinsic high-temperature superconductivity in ternary iron selenides

We examine superconductivity in the mesoscopically mixed antiferromagnetic(AF) and superconducting (SC) phases of ternary iron selenides K$_{y}$Fe$_{2-x}$Se$_{2}$. It is shown that the interlayer hopping and AF order are key factors to determine $T_{c}$ of the SC phase. In general, the hopping will produce deformed Fermi surfaces (FS's) that tend to suppress superconductivity. However, contrary to the common expectation, we find that larger AF order actually results in larger SC order, which explains the observed relatively high $T_{c}$ in these phases. Furthermore our results indicate that by reducing the interlayer hopping appropriately, phase-separated K$_{y}$Fe$_{2-x}$Se$_{2}$ may exhibit its intrinsic SC phase in the two dimensional limit with a much higher $T_{c}$ ($\sim 65K$) than what has been observed.

cond-mat.supr-con↗

arXiv2012

Grand-canonical variational approach for the t-J model

Gutzwiller-projected BCS wave function or the resonating-valence-bond (RVB) state in the 2D extended t-J model is investigated by using the variational Monte Carlo technique. We show that the results of ground-state energy and excitation spectra calculated in the grand-canonical scheme allowing particle number to fluctuate are essentially the same as previous results obtained by fixing the number of particle in the canonical scheme if the grand thermodynamic potential is used for minimization. To account for the effect of Gutzwiller projection, a fugacity factor proposed by Laughlin and Anderson few years ago has to be inserted into the coherence factor of the BCS state. Chemical potential, particle number fluctuation, and phase fluctuation of the RVB state, difficult or even impossible to be calculated in the canonical ensemble, have been directly measured in the grand-canonical picture. We find that except for La-214 materials, the doping dependence of chemical potential is consistent with experimental findings on several cuprates. Similar to what has been reported by scanning tunneling spectroscopy experiments, the tunneling asymmetry becomes much stronger as doping decreases. We found a very large enhancement of phase fluctuation in the underdoped regime.

cond-mat.str-el↗

arXiv2012

Matrix-Product based Projected Wave Functions Ansatz for Quantum Many-Body Ground States

We develop a new projected wave function approach which is based on projection operators in the form of matrix-product operators (MPOs). Our approach allows to variationally improve the short range entanglement of a given trial wave function by optimizing the matrix elements of the MPOs while the long range entanglement is contained in the initial guess of the wave function. The optimization is performed using standard variational Monte Carlo techniques. We demonstrate the efficiency of our approach by considering a one-dimension model of interacting spinless fermions. In addition, we indicate how to generalize this approach to higher dimensions using projection operators which are based on tensor products.

physics.comp-ph↗

arXiv2011

Nature of the inhomogeneous state of the extended t-J model on a square lattice

We carry out the variational Monte Carlo calculation to examine spatially inhomogeneous states in hole- and electron-doped cuprates. By using Gutzwiller approximation, we consider the excitations, arising from charge density, spin density and pair field, of the mean-field ground state of the t-J model. It leads to the stripe patterns we have found numerically in a generalized t-J-type model including mass renormalization from the electron-phonon coupling. In the hole-doped side, a robust d-wave superconducting order results in the formation of the half-doped antiferromagnetic resonating-valence-bond (AF-RVB) stripes shown by the well-known Yamada plot. On the other hand, due to a long-range AF order in electron-doped materials, a stripe structure with the "in-phase" magnetic domain (IPMD) is obtained in the underdoped regime instead of the AF-RVB stripe. The IPMD stripe with the largest period permitted by lattice size is stabilized near the underdoped region and it excludes the Yamada plot from electron-doped cases. Based on finite lattice size to which we can reach, the existence of IPMD stripes may imply an electronic phase separation into an electron-rich and an insulating half-filled AF long-range ordered domains in electron-doped compounds.

cond-mat.supr-con↗

arXiv2011

Enhanced Thermoelectric Power in Dual-Gated Bilayer Graphene

Thermoelectric power of a material, typically governed by its band structure and carrier density, can be varied by chemical doping that is often restricted by solubility of the dopant. Materials showing large thermoelectric power are useful for many industrial applications, such as the heat-to-electricity conversion and the thermoelectric cooling device. Here we show a full electric field tuning of thermoelectric power in a dual-gated bilayer graphene device resulting from the opening of a band-gap by applying a perpendicular electric field on bilayer graphene. We uncover a large enhancement in thermoelectric power at low temperature, which may open up a new possibility in low temperature thermoelectric application using graphene-based device.

cond-mat.mes-hall↗

arXiv2009

Mechanism of formation of half-doped stripes in underdoped cuprates

Using a variational Monte-Carlo approach with a recently proposed stripe wave function, we showed that the strong correlation included in a t-J-type model has essentially all the necessary ingredients to form these stripes with modulations of charge density, spin magnetization, and pair field. If a perturbative effect of electron-phonon coupling to renormalize the effective mass or the hopping rate of holes is considered with the model, we find the half-doped stripes, which has on the average one half of a hole in one period of charge modulation, to be most stable, energetic wise in the underdoped region, $1/12\leqδ\leq1/8$. This is in good agreement with the observation in the neutron scattering experiments. We also find long range Coulomb interaction to be less effective in the formation of half-doped stripes.

cond-mat.supr-con↗

arXiv2005

Numerical study of magnetic field induced ordering in BaCuSi$_2$O$_6$ and related systems

Thermodynamics of spin dimer system BaCuSi_2O_6 is studied using a quantum Monte Carlo calculation (QMC) and a bond-operator mean field theory. We propose that a new type of boson, which, rather than being hard-core, allows up to two occupancy at each site, is responsible for the Bose Einstein condensation of field induced ordering. Its superfluid density is identified as the square of the in-plane staggered magnetization m_{xy} in the ordered phase. We also compare our QMC result of the spin Heisenberg model to those predicted by mean field theory as well as by the simple hard core boson model for both large and small intra-dimer coupling J. The asymmetry of the phase diagram of m_{xy}(h) of small coupling J in related systems such as NiCl_2-4SC(NH_2)_2 is explained with our new boson operator.

cond-mat.other↗